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Genetic basis for lactose intolerance revealed

A quick and cheap genetic test will soon be able to identify people with lactose intolerance. The test will be a boon for doctors, since many people suffer from the condition without realising it, and existing tests are time-consuming and unreliable.

For perhaps the majority of people in the world, including most southern European, Asian and African populations, lactose intolerance is the norm. It sets in at weaning or shortly after, when the body stops producing lactase – the enzyme it needs to digest the sugar lactose, which is a major ingredient of human and animal milk.

Without lactase, lactose passes through the stomach undigested and reaches bacteria in the large intestine. There some bugs feast on it, belching out by-products that can leave people feeling gassy and nauseous, or worse.

Now Leena Peltonen’s team at the University of California, Los Angeles, has discovered the genetic basis for lactose intolerance. The discovery supports the theory that retaining the ability to digest milk evolved only in some peoples in the past ten thousand years, as an adaptation to dairy farming.

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Drinking milk

Peltonen’s team studied nine extended Finnish families, as well as some Germans, Italians and South Koreans. The researchers found two variations in the human genome associated with lactose intolerance.

One of these “single nucleotide polymorphisms”, or SNPs, was present in all 236 people who were lactose intolerant, while the other was found in 229. Both SNPs are near the lactase gene, and probably affect proteins that regulate the expression of the gene.

The fact that the same variations occur in distantly related populations supports the theory that all humans were once lactose intolerant, and that “lactase persistence” evolved only after people domesticated animals and began drinking their milk.

Original condition

Lactase persistence also seems to be most common among peoples with a long tradition of dairy farming, such as northern Europeans, some groups in India and the Tutsis in central Africa. “I find it ironic that a so-called disease actually represents the original condition,” says Peltonen.

It is a nice example of a genetic change prompted by a cultural practice, says Kevin Laland, an expert on the interaction between genetics and culture at Cambridge University. “There are bound to be thousands of such changes, but there are comparatively few where the gene has been isolated.”

The widespread prevalence of lactose intolerance was only recognised in the 1960s. Before that, a dislike of milk in countries such as China was ascribed to cultural differences.